1. Field of the Invention
The present invention relates to an X-ray analyzer in which secondary X-rays, generated when primary X-rays from an X-ray source are incident upon a sample, are projected by an X-ray detector and elements contained in the sample can then be analyzed on the basis of an output from the X-ray detector, and more particularly, to a self-correcting feature to improve and maintain the accuracy of the output readings.
2. Description of Related Art
A conventional X-ray analyzer has a construction as shown in, for example, FIG. 6. In this FIG., reference No. 1 designates a sample, reference No. 2 designates an X-ray tube as one example of an X-ray source adapted to emit primary X-rays 3 towards the sample 1. A proportional counter 4 is one example of an X-ray detector capable of detecting the secondary X-ray 7, such as, for example, fluorescent X-rays 5 and scattered X-rays 6, that are emitted from the sample 1 when the primary X-rays 3 are incident upon the sample 1. These secondary X-rays are characteristic of the elements contained in the sample 1. The proportional counter 4 is operated by applying a predetermined high voltage from a high voltage source 8 and is capable of outputting an (eV) quantity of charges proportional to the energy of the detected X-rays.
An amplifier 9 can suitably amplify the output from the proportional counter 4, and this output can then be converted by an A-D converter to convert the analog signal put out from the amplifier 9 into a digital signal. A multi-channel analyzer 11 (hereinafter referred to as MCA) is capable of memorizing a predetermined number of output signals from the A-D converter 10 and to carry out a statistical analysis of these stored signals. A CPU 12 is capable of processing the signal output from the MCA 11 to determine the constituent elements in the sample.
In an X-ray analyzer of the conventional configuration, an energy spectrum is shown, for example, in FIG .7 representing the output of the detector over an energy band of the detected X-rays, after the primary X-rays 3 were incident upon the sample 1. In FIG. 7, the mark (a) designates the detected count of fluorescent X-rays 5 of an element contained in the sample, while the mark (b) designates the detected count of the scattered X-rays 6. As can be seen, the X-rays designated by marks (a), (b) have energy levels characteristic of the individual elements of the sample and disclose a Gaussian distribution or a distribution corresponding to a substantially Gaussian distribution having peaks (a.sub.p), (b.sub.p), at specific positions across the energy spectrum. Based on this information, the element contained within the sample can be determined from the position of the peak (a.sub.p) and a concentration of the element contained within the sample can be measured from the total number of counts within an energy range (a.sub.1 to a.sub.2) of the fluorescent X-rays 5. As can be appreciated, with such an X-ray analyzer, even though a sample may have a diverse number of elements contained therein, they can be sufficiently analyzed together along with their specific concentrations.
A problem can exist in such an X-ray analyzer having the above construction as a result of the effects of temperature and time on the performance of the electronic components that are utilized in the circuit, for example, if the proportional counter 4, the amplifier 9, or the A-D converter 10 changes in temperature, or even in some cases over a lapse of a period of time, then a signal gain that is determined through these elements can drift, and an error can be produced in the counting number of the fluorescent X-rays 5. If, for example, a signal gain is reduced, the energy spectrum, as shown by the full line in FIG. 8(a), is obtained. Alternatively, if the signal gain is increased, the energy spectrum, as shown by the full line in FIG. 8(b), would be obtained. As can be readily appreciated, in both examples, the energy spectrum has been greatly shifted from the desired energy spectrum, as shown by the dotted, imaginary lines in both figures and the corresponding position of the peaks (a.sub.p), (b.sub.p), also are shifted, resulting in an error in the analysis of the elements contained in the sample.
In order to avoid such a deterioration in the accuracy of measurement resulting from any shift in the peak positions (a.sub.p), (b.sub.p), that would occur, as mentioned above; attempts have been made to determine the peak position through an analysis of the output signals in a CPU 12. This method, however, requires a significant amount of data to be stored so that not only will a real time analysis be delayed, but also a problem has occurred in that the complexity of operation of the CPU 12 is significantly increased with both cost and the possibilities of error in the calculation being apt to occur.
The above described problems have also occurred in an X-ray analyzer using a Si (Li) detector and a Ge detector other than the proportional counter and also with an X-ray analyzer using a window comparator and a single-channel analyzer in place of the A-D converter 10 and the MCA 11, respectively. Thus, the prior art is still seeking to optimize, in an economical manner, the accuracy of an X-ray analyzer.